feat(signal_v2): stock_client + 6 integration tests

httpx async client with custom retry loop (max 3, exponential 1s/2s/4s),
memory dict cache (portfolio 60s / news-sentiment 300s / screener 60s),
X-WebAI-Key auth header injection. Stale fallback returns last
successful response with logger.warning on persistent failures.

6 integration tests pass with respx httpx mock.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

signal_v2/tests/conftest.py

@@ -0,0 +1,18 @@
+"""Pytest fixtures for signal_v2 tests."""
+from pathlib import Path
+
+import pytest
+import respx
+
+
+@pytest.fixture
+def tmp_dedup_db(tmp_path) -> Path:
+    """SQLite 단위 테스트용 임시 DB path."""
+    return tmp_path / "test_signal_v2.db"
+
+
+@pytest.fixture
+def mock_stock_api():
+    """respx 로 stock API mock. base_url 은 테스트마다 임의."""
+    with respx.mock(base_url="https://test.stock.local", assert_all_called=False) as mock:
+        yield mock